In orthodontology, the most normal method used when replacing lost or decayed tooth substance in a single tooth, or when replacing teeth which have been lost for some reason or other, is one in which the replacement substance or material is cast or molded in an alloy of gold. In the former case, the lost tooth substance is replaced with a gold insert (minor losses of tooth substance), while in the latter case, the replacement is in the form of a crown prosthesis which may be of varying design, such as full crowns and partial crowns. Full crowns may be made entirely of metal. As the name implies, the partial crowns only replace a part of the crowns of the teeth. Both full crowns and partial crowns are designed to include the remaining parts of the tooth crown or crowns. Separate gold fillings and individual crown prostheses, and also bridge prostheses, are called permanent fixtures or replacements since they are normally cemented to the teeth and cannot be removed without being broken.
Before a tooth can be filled with gold or fitted with a crown prosthesis, it must first be shaped and prepared. Existing guidelines and rules relating to the preparatory work involved are well known to those skilled in this art and do not constitute any part of the present invention. Consequently, they will not be dealt with in detail here. When dental gold work is cast or moulded, there is first always formed a wax model of the teeth being repaired, which is then embedded in a mould and the wax is subsequently replaced with a gold alloy. The wax model is most often produced by the so-called indirect method, by which is meant that all, or the greater part, of the dental work involved is carried out on a mould, i.e., an impression of the relevant part of the oral cavity. Such a model, referred to as the preparation model, is often not sufficient in itself to enable dental gold work to be carried out, since it must also be capable of being placed, in one way or another, in correct relationship with surrounding and corresponding parts of the bite. Consequently, a working model incorporating the copy of the prepared teeth is normally produced.
The actual casting process, as applied today, incorporates a number of working stages comprising the following
(a) forming in the wax a runner through which the casting metal can be poured, or fixing a metal runner to the wax model; PA1 (b) embedding the wax model together with the runner in a heat-resistant bedding mass; PA1 (c) heating the embedding mass to eliminate the wax and runner therefrom; PA1 (d) permitting the molten metal to fill the mould freed from wax, via mould craters and runners; and PA1 (e) working the solidified metal, for example, grinding and polishing the same and applying surface material to the thus worked surfaces.
It will be evident herefrom that the work involved in replacing lost tooth substance is extremely complicated. It is also a known fact to those skilled in this art that the casting process places high demands on the expertise and accuracy of the person carrying out the work.
Different types of cast gold constructions place different requirements on the hardness and mechanical strength of the gold alloy used. For example, while a buccal gold filling, which is subject to no load or only an insignificant load, can be cast advantageously from a soft alloy, the alloy used to build a bridge, a crown or a filling subjected to heavy pressure by the teeth must be much harder and of much greater mechanical strength. The chewing habits and abrasion tendencies of the patient are two factors which influence the choice of the casting alloy.
In recent years the use of hard-gold alloys has progressively increased, the mechanical and electrochemical properties of these alloys being improved by admixing metals from the platinum metal group.
Irrespective of the alloy used, its gold content is normally at least 70%. In recent time, however, doubt has been expressed as to whether, from a biological aspect, gold can be considered an acceptable material in this respect. It has also been found that copper, palladium etc., are precipitated, which can result in subsidiary effects, at least in the long term.
Furthermore, gold is, in itself, a very expensive metal which together with the relatively complicated casting process required renders the total cost for an insert, filling, crown or the like highly expensive.